1. Field of the Invention
The present invention relates to a reading apparatus for reading an image recorded on a film by means of photoelectric conversion of transparence light quantity or reflection light quantity of the film exposed or illuminated by a light source.
2. Related Background Art
Microfilming of documents has been widely employed for reducing the space required for storing a large amount of documents or facilitating data retrieval.
On the other hand, owing to recent technical progress in electrical image processing, it has been proposed to read a microfilmed image with an image sensor with a photoelectric converting function such as a CCD image sensor and to effect display, recording, storage or transmission of the image based on thus the obtained image signal.
Reading of a microfilmed image is generally conducted by illuminating a desired frame of microfilm with a light source, and focusing the transmitted light onto an image sensor for photoelectric conversion through an optical system such as a lens or a mirror. Consequently the density of the read image is influenced by the intensity of the light source illuminating the microfilm, and this intensity has to be regulated to an optimum value in order to achieve satisfactory image reading.
Also the light image has to be exactly focused onto the light-receiving face of the image sensor since otherwise, the obtained output will become blurred.
Furthermore, since the contrast of the microfilmed image is variable depending on the photographic conditions at recording and the film characteristics, a satisfactory image reading cannot be expected if the image signal from the image sensor is always processed in the same manner.
For this reason, the present applicant disclosed, in the U.S. patent application Ser. No. 878,790 a technology of regulating the amount of light of the light source for illuminating the film to a predetermined value upon detecting the amount of light. Also we disclosed, in the U.S. patent application Ser. No. 854,569 an automatic focusing technology in which the focusing is automatically conducted in accordance with a light image transmitted by a film. Furthermore we disclose,, in the U.S. patent application Ser. No. 878,790 a technology of varying the process for the image signal obtained from an image sensor, in accordance with measurement of the microfilmed image density.
However, these three technologies are in fact not independent. For example, the automatic focusing cannot be executed satisfactorily if the illuminating amount of light is not adequate. Also an inexact focusing will result in an incorrect measurement of image density, in turn leading to an improper automatic exposure operation.